Methods and apparatus for providing a sufficiently stable power to a load in an energy transfer system

ABSTRACT

Methods and apparatus for providing a sufficiently stable power to a load in an energy transfer system that transfers energy from one side of a physical boundary to another side of he boundary. In one example, a power supply and a primary winding are located on a first side of a physical boundary (e.g., external to a body), and a secondary winding and the load are located on a second side of the physical boundary (e.g., internal to the body). A primary voltage across the primary winding is regulated so as to provide a sufficiently stable output power to the load notwithstanding changes in the load and/or changes in a relative position of the primary winding and the secondary winding. One aspect of the invention relates to energy transfer methods and apparatus for use in connection with the human body. In particular, one example of the invention includes a transcutaneous energy transfer (TET) system for transferring power from a power supply external to the body to a device implanted in the body.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit, under 35 U.S.C.§119(e), of U.S. Provisional Application Serial No. 60/160,343, filedOct. 19, 1999, entitled ADAPTIVE TET SYSTEM, which application is herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to energy transfer methods andapparatus. More particularly, the invention relates to providing asufficiently stable power to a load in an energy transfer system thattransfers power across a physical boundary (e.g., from a power supplylocated external to a body to a load located internal to the body).

[0004] 2. Related Art

[0005] In a variety of scientific, industrial, and medically relatedapplications, it may be desirable to transfer energy or power (energyper unit time) across some type of boundary. For example, one or moredevices that require power (e.g., electrical, mechanical, optical, andacoustic devices) may be located within the confines of a closed system,or “body,” in which it may be difficult and/or undesirable to alsoinclude a substantial and/or long term source of power. The closedsystem or body may be delimited by various types of physical boundaries,and the system internal to the boundary may be living or inanimate, mayperform a variety of functions, and may have a variety of operationaland physical requirements and/or constraints. In some cases, suchrequirements and constraints may make the implementation of asubstantial and/or long term “internal” power source for internallylocated devices problematic.

[0006] In some closed systems, repeated entry. into the system may beundesirable for a variety of reasons. In other closed systems,significant internal power requirements and a limited internal space mayprohibit the implementation of a suitably sized internal power source.In yet other systems, contamination and/or security issues may poseparticular challenges in implementing an internal power source. For anycombination of the foregoing and other reasons, a power source externalto the system and some feasible means of transferring power from theexternal source to one or more internal devices may be preferable insome applications.

[0007] One example of a closed system is the human body. In somemedically related and scientific applications, a variety of prostheticand other devices that require power may be surgically implanted withinvarious portions of the body. Some examples of such devices include asynthetic replacement heart, a circulatory blood pump or ventricularassist device (VAD), a cochlear (ear) implant, a pacemaker, and thelike. With respect to the human body, issues such as repeated reentry orsurgery, internal space limitations, and contamination (e.g., infection)may make the implementation of a suitable internal power source for someof these implanted devices impractical.

[0008] Accordingly, in some medical implant applications,“transcutaneous energy transfer” (TET) devices are employed to transferenergy from outside the body to inside the body, to provide power to oneor more implanted prostheses or devices from an external power source.One example of a conventional TET device is a transformer that includesa primary winding (or coil) external to the body and a secondary windinginternal to the body. Both the primary and secondary windings generallyare placed proximate to respective outer and inner layers of a patient'sskin; hence, the term “transcutaneous” commonly refers to energytransfer “through the skin.” Energy is transferred from the primarywinding to the secondary winding in the form of a magnetic field. Theimplanted secondary winding converts the transferred energy in themagnetic field to electrical power for the implanted device, which actsas a “load” on the secondary winding.

[0009] In general, TET devices differ from conventional powertransformers in that power transformers typically include a magneticcore around which the primary and secondary windings are wound, thusfixing the relative positions of the primary and secondary windings. Incontrast, the primary and secondary windings of conventional TET devicesare not necessarily fixed in position with respect to one another.Accordingly, one issue associated with conventional TET devices is thatthe power supplied by the secondary winding to a load (e.g., animplanted device) may be quite sensitive to more than nominal or trivialphysical displacements of either the primary winding or the secondarywinding from an optimum coupling position. The resolution of this issuedetermines the suitability of the TET technology to a particular type ofload.

[0010] For example, implanted prostheses or other devices, andparticularly an implanted device that performs a life sustainingfinction, generally must have a consistent source of available power.Without a consistent power source, the implanted device may functionerratically or intermittently. Such an erratic or intermittent operationcan have undesirable, and in some cases serious life threatening effectson the patient. Accordingly, with TET devices in particular, and otherenergy transfer systems in general which transfer energy across aboundary, it is desirable to accurately and reliably provide asufficiently stable power to the load.

SUMMARY OF THE INVENTION

[0011] One embodiment of the invention is directed to an apparatus foruse in a transcutaneous energy transfer system that includes a powersupply, a primary winding and a secondary winding. The apparatuscomprises a control circuit, coupled to the power supply and the primarywinding, to monitor a primary amplitude of a primary voltage across theprimary winding and to control the primary amplitude based only on themonitored primary amplitude and a reference voltage.

[0012] Another embodiment of the invention is directed to a method in atranscutaneous energy transfer system including a power supply, aprimary winding and a secondary winding. The method comprises acts ofmonitoring a primary amplitude of a primary voltage across the primarywinding, and regulating the primary amplitude based only on themonitored primary amplitude and a reference voltage.

[0013] Another embodiment of the invention is directed to an energytransfer system for transferring power from a power supply located on afirst side of a physical boundary to a variable load located on a secondside of the physical boundary. The energy transfer system comprises aprimary winding electrically coupled to the power supply to generate amagnetic field based on input power provided by the power supply,wherein the magnetic field permeates the physical boundary. The systemalso comprises a secondary winding, magnetically coupled to the primarywinding via the magnetic field, to receive at least a portion of themagnetic field. The secondary winding is electrically coupled to thevariable load to provide output power to the variable load based on thereceived magnetic field. The system also comprises at least one controlcircuit, electrically coupled to at least the primary winding, toregulate a primary voltage across the primary winding such that asufficiently stable output power is provided to the variable loadnotwithstanding at least one of changes in the load and changes in arelative position of the primary winding and the secondary winding.

[0014] Another embodiment of the invention is directed to a method oftransferring power from a power supply to a variable load in an energytransfer system that includes a primary winding electrically coupled tothe power supply located on a first side of a physical boundary, and asecondary winding electrically coupled to the variable load located on asecond side of the physical boundary. The method comprises an act ofregulating a primary voltage across the primary winding so as to providea sufficiently stable output power to the variable load notwithstandingat least one of changes in the variable load and changes in a relativeposition of the primary winding and the secondary winding.

[0015] Another embodiment of the invention is directed to an apparatusfor use in an energy transfer system, wherein the energy transfer systemincludes a power supply electrically coupled to a primary winding, and asecondary winding magnetically coupled to the primary winding. Theprimary and secondary windings are used to transfer power from the powersupply to a load that is electrically coupled to the secondary winding.The primary winding and the secondary winding do not have a fixedspatial relationship to each other. The apparatus comprises a secondarycircuit, electrically coupled to the secondary winding, to monitor ameasurable quantity associated with the load and to provide a detectableindication based on the monitored measurable quantity. The apparatusalso comprises a primary circuit, electrically coupled to the primarywinding, to monitor the detectable indication provided by.the secondarycircuit and to regulate a primary voltage across the primary windingbased on the detectable indication so as to regulate a load voltageacross the load.

[0016] Another embodiment of the invention is directed to a method in atranscutaneous energy transfer system that includes a power supplyelectrically coupled to a primary winding and a secondary windingmagnetically coupled to the primary winding. The primary and secondarywindings are for transferring power from the power supply to a load thatis electrically coupled. to the secondary winding. The primary windingand the secondary winding do not have a fixed spatial relationship toeach other. The method comprises acts of making a comparison of ameasurable quantity associated with the load and a predeterminedthreshold level, activating a secondary circuit to provide a detectableindication based on the comparison, and regulating a primary voltageacross the primary winding based on the detectable indication.

[0017] Another embodiment of the invention is directed to an energytransfer system for transferring power from a power supply located on afirst side of a physical boundary to a variable load located on a secondside of the physical boundary. The energy transfer system comprises aprimary winding electrically coupled to the power supply to generate amagnetic field based on input power provided by the power supply, and asecondary winding magnetically coupled to the primary winding to receiveat least a portion of the magnetic field generated by the primarywinding. The secondary winding provides output power to the variableload based on the received magnetic field, and the magnetic field formsa portion of a power channel between the primary winding and thesecondary winding to transfer at least some of the power from the powersupply to the variable load. The energy transfer system also comprises afirst control circuit, electrically coupled to the primary winding, toregulate a load voltage across the variable load based on informationrelated to the variable load that is obtained via the power channel.

[0018] Another embodiment of the invention is directed to a method in anenergy transfer system for transferring power from a power supplylocated on a first side of a physical boundary to a variable loadlocated on a second side of the physical boundary. The energy transfersystem includes a primary winding electrically coupled to the powersupply to generate a magnetic field based on input power provided by thepower supply, and a secondary winding magnetically coupled to theprimary winding to receive at least a portion of the magnetic fieldgenerated by the primary winding. The secondary winding provides outputpower to the variable load based on the received magnetic field, and themagnetic field forms a portion of a power channel between the primarywinding and the secondary winding to transfer at least some of the powerfrom the power supply to the variable load. The method comprises acts ofmonitoring the power channel to obtain information related to thevariable load, and regulating a load voltage across the variable loadbased on the information obtained via the power channel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings are not intended to be drawn to scale.In the drawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing.

[0020]FIG. 1 is a block diagram showing an example of an energy transfersystem according to one embodiment of the invention;

[0021]FIG. 2 is a more detailed block diagram of the energy transfersystem shown in FIG. 1, according to one embodiment of the invention;

[0022]FIGS. 3A, 3B, and 3C are graphs that each show a plot of a primaryvoltage across a primary winding of the energy transfer system of FIG. 2as a function of time, and a corresponding current through a load,according to one embodiment of the invention;

[0023]FIG. 3D is a circuit diagram showing one example of an electroniccircuit implementation for a primary side of the energy transfer systemof FIG. 2, according to one embodiment of the invention;

[0024]FIG. 4 is a block diagram of an energy transfer system accordingto another embodiment of the invention;

[0025]FIG. 5 is a more detailed block diagram of the energy transfersystem shown in FIG. 4, according to one embodiment of the invention;

[0026]FIG. 6 is a diagram showing an example of a detectable indicationon a power channel of the energy transfer system of FIG. 5, whichindication is used to regulate voltages in the energy transfer system,according to one embodiment of the invention;

[0027]FIG. 7 is a graph showing plots of various signals in the energytransfer system of FIG. 5, according to one embodiment of the invention;

[0028]FIGS. 8A and 8B are graphs that each show plots of a load voltageand a current through a, primary winding in the energy transfer systemof FIG. 5, according to one embodiment of the invention;

[0029]FIG. 9 is a circuit diagram showing one example of an electroniccircuit implementation for a secondary side of the energy transfersystems of FIGS. 2 or 5, according to one embodiment of the invention;

[0030]FIG. 10 is a circuit diagram showing one example of an electroniccircuit implementation of both primary and secondary sides of the energytransfer system shown in FIG. 5, according to one embodiment of theinvention; and

[0031]FIG. 11 is a circuit diagram showing another example of anelectronic circuit implementation of the primary side of the energytransfer system of FIG. 5, according to one embodiment of the invention.

DETAILED DESCRIPTION

[0032] One embodiment of the present invention is directed to methodsand apparatus for providing a sufficiently stable power to a load in anenergy transfer system. In one aspect of this embodiment, the energytransfer system may transfer energy across a physical boundary. Theboundary may be formed, for example, by some inanimate material orliving tissue and may be in a solid, liquid or gaseous phase. Theboundary may also be formed by a combination of inanimate materials orliving tissue in various phases. In another aspect, the boundary may bearranged to form a closed system (e.g., the boundary may completelysurround a “body”), and the energy transfer system may be implemented soas to transfer energy, from a position external to the body, across theboundary to a position internal to the body. The transferred energy maybe used to provide power to one or more devices located internal to-thebody. One example of an application of such an energy transfer system isproviding power to one or more devices implanted in a human body, asdiscussed further below.

[0033] As shown in FIG. 1, energy transfer systems according to variousembodiments of the invention typically include a power supply 20 and aprimary winding 32 electrically coupled to the power supply to generatea magnetic field 36 based on input power provided by the power supply.Energy transfer systems according to the invention also typicallyinclude a secondary winding 34 that is magnetically coupled to theprimary winding 32 via the magnetic field 36. In FIG. 1, the magneticfield 36 is symbolically illustrated as a number of arrows emanatingfrom the primary winding 32 and propagating toward the secondary winding34.

[0034] According to one embodiment, the primary winding 32 and thesecondary winding 34 shown in FIG. 1 are positioned relative to eachother such that the secondary winding 34 receives at least a portion ofthe magnetic field 36. The secondary winding 34 is electrically coupledto a load 44 and provides output power to the load 44 based on thereceived magnetic field. In various embodiments, the load 44 mayrepresent one or more of a variety of devices that use the output powerprovided by the secondary winding 34 to perform some function. The loadgenerally is associated with some resistance or impedance that, in someapplications, may vary from time to time during normal operation of theload depending, in part, on the particular function that the load isperforming. In this manner, the load may be a variable load (as shownsymbolically in FIG. 1, for example, by a variable resistor).

[0035] According to one embodiment of the invention, the power supply 20and the primary winding 32 shown in FIG. 1 are located on a first sideof a physical boundary (e.g., external to a body), and the secondarywinding 34 and the load 44 are located on a second side of the physicalboundary (e.g., internal to the body). As discussed above, some energytransfer systems that transfer energy across a boundary (e.g., TETsystems) include primary and secondary windings that are not necessarilyphysically coupled to each other in a rigidly fixed manner (i.e., somemovement of the windings relative to each other is possible).Accordingly, these energy transfer systems may be susceptible to outputpower fluctuations due not only to changes in the load from time to time(i.e., varying load resistance or impedance), but also due to changes inrelative position of the at primary and secondary windings. For purposesof the present disclosure, the terms “changes in relative position”refer to one or more of an axial displacement between the primary andsecondary windings (as indicated, for example, by the referencecharacter 38 in FIG. 1), a lateral displacement between the windings ina direction essentially orthogonal to the axial displacement 38 (asindicated, for example, by the reference character 39 in FIG. 1), andany change in orientation of one winding (e.g., rotation of a windingabout an axis of rotation) relative to the other winding (as indicated,for. example, by the reference characters 41A and 41B in FIG. 1).

[0036] In view of the foregoing, in one aspect of the invention, theenergy transfer system is controlled so as to provide a sufficientlystable power to the load notwithstanding one or both of changes in theload from time to time (i.e., varying load resistance or impedance), andchanges in relative position of the primary and secondary windings. Inparticular, in some embodiments, the power ultimately provided to theload may be controlled by regulating various voltages in the system.

[0037] For example, in one illustrative embodiment, as shown in FIGS. 1and 2 and discussed in greater detail below, a primary amplitude of aprimary voltage across the primary winding 32 of the energy transfersystem is regulated such that the primary amplitude is held essentiallyat some constant value that is proportional to a fixed referenceparameter (e.g., a reference voltage). In this embodiment, the primaryamplitude is regulated essentially independently of system conditionsassociated with the secondary winding 34 and any circuitry coupled tothe secondary winding (including the load). In particular, in one aspectof this embodiment, regulation of the primary amplitude does notnecessarily depend upon changes in the load and changes in relativeposition of the windings. Instead, by selecting an appropriate referenceparameter, the primary amplitude is regulated at essentially a constantvalue such that a sufficiently stable power is provided to the load overa predetermined range for the load voltage, which voltage may nominallyfluctuate due to changes in the load and/or changes in relative windingposition. At least one favorable feature of this embodiment issimplicity of implementation while nonetheless providing sufficientcontrol of the energy transfer system.

[0038] In another exemplary embodiment of the invention, as illustratedin FIGS. 4 and 5 and discussed in greater detail below, the primaryamplitude of the voltage across the primary winding 32 is varied in acontrolled manner, based at least in part on changes in the load and/orchanges in the relative position of the primary and secondary windingsof the energy transfer system, such that the load voltage is regulatednear a predetermined threshold value. Accordingly, the embodiment ofFIGS. 4 and 5 differs from that of FIGS. 1 and 2 at least with respectto the manner of regulation of the primary amplitude. In one aspect ofthis embodiment, the primary amplitude is controlled based oninformation related to the load that is obtained via a power channel ofthe energy transfer system. In some aspects, the embodiment of FIGS. 4and 5 provides a more sophisticated implementation of an energy transfersystem than does the embodiment of FIGS. 1 and 2, by regulating the loadvoltage within a relatively narrower range of voltages. However, itshould be appreciated that both of the embodiments discussed above, aswell as other embodiments of the invention, provide viable alternativesolutions for a variety of energy transfer system control applications.

[0039] Another aspect of the present invention relates to the use ofvarious embodiments of energy transfer methods and apparatus of theinvention (including the above-described embodiments) in connection withthe human body. In particular, one embodiment of the invention isdirected to a transcutaneous energy transfer (TET) system fortransferring power from a power supply external to the body to a deviceimplanted in the body. Such an implementation places stringentrequirements on the ability of the TET system to accurately and reliablyfunction under significant variations in load and/or relative positionof the primary winding 32 and the secondary winding 34. In one aspect ofthis embodiment, the variable load 44 indicated in various figures mayinclude one or more prosthetic devices implanted in a human body.Examples of such devices include, but are not limited to, artificialhearts, ventricular assist devices, cardioverter/defibrillators,infusion pumps, pacemakers, cochlear implants, and the like. In additionto any of the foregoing examples, the variable load 44 also may includea rechargeable battery that is used, for example, as a temporary backuppower source in the event that power provided by the power source 20 isinterrupted.

[0040] Following below are more detailed descriptions of variousconcepts related to, and embodiments of, methods, apparatus, and systemsaccording to the present invention for providing a sufficiently-stablepower to a load in an energy transfer system. It should be appreciatedthat various aspects of the invention as discussed herein may beimplemented in any of numerous ways, as the invention is not limited toany particular manner of implementation. Examples of specificimplementations are provided for illustrative purposes only.

[0041]FIG. 1 shows that, in one embodiment, an energy transfer system ofthe invention includes a primary circuit 22, electrically coupled to theprimary winding 32, to regulate a primary voltage across the primarywinding 32. In FIG. 1, the voltage across the primary winding 32 isillustrated symbolically by an approximately sinusoidal waveform; itshould be appreciated, however, that the invention is not limited inthis respect, as other voltage waveforms may be suitable in variousembodiments. For purposes of the following discussion, the power supply20, the primary circuit 22, the primary winding 32, and any othercomponents electrically coupled to the primary winding 32 may bereferred to collectively as a “primary side” of the energy transfersystem.

[0042]FIG. 1 also shows that, according to one embodiment, an energytransfer system of the invention includes a secondary circuit 42electrically coupled to the secondary winding 34 and the load 44, toprovide output power to the load. For purposes of the followingdiscussion, the secondary circuit 42, the secondary winding 34, the load44, and any other components electrically coupled to the secondarywinding 34 may be referred to collectively as a “secondary side” of theenergy transfer system.

[0043] In the embodiment of FIG. 1, the primary circuit 22 includes oneor more capacitors 30 coupled to the primary winding 32 to form aprimary resonant circuit 48 with the primary winding 32 (the primaryresonant circuit 48 is indicated in FIG. 1 with dashed lines). In thisembodiment, the primary circuit 22 also includes an excitation circuit24, electrically coupled to the power supply 20 and the primary resonantcircuit 48, to provide input power to the primary resonant circuit.Additionally, the primary circuit 22 of this embodiment includes anexcitation control circuit 26, electrically coupled to the excitationcircuit 24 and the primary winding 32, to control the excitation circuit24 so as to modulate the input power provided to the primary resonantcircuit 48.

[0044] In one aspect of the embodiment of FIG. 1, the primary circuit 22regulates the primary amplitude 46 of the voltage across the primarywinding 32 such that the primary amplitude is held essentially constant.In particular, the primary circuit 22 regulates the primary amplitude 46by modulating the input power provided by the power supply 20 to theprimary winding 32, such that the primary amplitude is maintainedapproximately at a value that is proportional to a fixed referenceparameter. As can be seen in FIG. 1, the primary side of the energytransfer system is configured as a feedback loop that includes theexcitation control circuit 26 and the excitation circuit 24. A signalassociated with the primary winding 32 is monitored as one input 25 tothe feedback loop. The monitored input signal is compared to a referenceparameter 27 to effect control (e.g., modulation) of the input powerprovided to the primary resonant circuit. More specifically, theexcitation control circuit 26 controls the excitation circuit 24 basedon a comparison of a reference parameter 27 and a measurablecharacteristic of a signal associated with the primary winding 32.

[0045] In the embodiment illustrated in FIG. 1, the reference parameter27 is generated by a fixed reference source 28. In one aspect of theembodiment shown in FIG. 1, the reference parameter 27 may be, forexample, a reference voltage or a reference current provided bythe-fixed reference source 28. It should be appreciated, however, thatthe invention is not limited in this respect; namely, in anotherembodiment, the reference parameter 27 may be provided by a varyingreference source (as discussed further below in connection with FIGS. 4and 5).

[0046] Additionally, the measurable characteristic of a signalassociated with the primary winding 32 (which is monitored as an input25 to the primary side feedback loop of FIG. 1) may include, forexample, the primary amplitude 46 of the primary voltage across theprimary winding 32. However, it should be appreciated that the inventionis not limited in this respect, as other measurable characteristics(e.g., current, frequency, phase) of a signal associated with theprimary winding 32 may be compared by the excitation control circuit 26to one or more of a variety of reference parameters 27 for purposes ofcontrolling the excitation circuit 24, according to other embodiments ofthe invention.

[0047] In the embodiment of FIG. 1, the primary amplitude 46 may not beheld precisely at a fixed value at all times by the excitation controlcircuit 26 and the excitation circuit 24, since feedback controlgenerally is associated with some response time. In particular, theprimary side feedback loop generally must sense some change in a singalmonitored at the input 25 before effecting some control function.However, for purposes of the present discussion, it should beappreciated that the primary amplitude 46 is essentially maintained at aconstant value that is proportional to the fixed reference parameter 27as a result of the feedback control loop, with some nominal variationfrom the regulated value due to a response time of the feedback loop.

[0048] In one embodiment, the excitation circuit 24 of FIG. 1 outputsone or more pulses 50 to the primary resonant circuit 48 to provide theinput power to the primary resonant circuit. In one aspect of thisembodiment, the excitation control circuit 26 controls a width 52 of thepulses 50 output by the excitation circuit 24, such that the measurablecharacteristic of the signal associated with the primary winding 32 isapproximately equal to a predetermined value that is proportional to thereference parameter 27. For example, in one aspect, the excitationcontrol circuit 26 controls the width 52 of the pulses 50 such that theprimary amplitude 46 is approximately equal to a predetermined amplitudethat is proportional to the reference parameter 27 (e.g., a referencevoltage).

[0049]FIG. 2 is a more detailed block diagram of the energy transfersystem shown in FIG. 1, according to one embodiment of the invention. Inthe embodiment of FIG. 2, the excitation control circuit 26 monitors (atthe input 25) the primary amplitude 46 of the primary voltage across theprimary winding 32. Additionally, the excitation control circuit 26controls the excitation circuit 24, based only on the monitored primaryamplitude and the reference parameter 27 (e.g., a reference voltage), soas to regulate the primary amplitude 46 of the voltage across theprimary winding 32 (which forms part of the primary resonant circuit 48shown in FIG. 1). In the embodiment of FIG. 2, the reference parameter27 is selected such that the primary amplitude 46 is regulated toprovide a sufficiently stable output power to the variable load 44notwithstanding changes in the load and/or relative position of theprimary and secondary windings, as discussed further below. FIG. 2 alsoidentifies a load voltage 45 (V_(load)) across the load 44 and a loadcurrent 74 through the load 44.

[0050] In one aspect of the embodiment shown in FIG. 2, the excitationcontrol circuit 26 controls the excitation circuit 24 such that theprimary voltage has essentially a constant frequency 54. The constantfrequency 54 may be selected based on a resonant frequency of theprimary resonant circuit formed by one or more capacitors 30 and theprimary winding 32. In one embodiment, one or more capacitors 30 areselected such that the resonant frequency of the primary resonantcircuit approximates that of a resonant frequency of a secondaryresonant circuit formed by the secondary winding 34 and one or morecapacitors 40 coupled to the secondary winding, as discussed furtherbelow.

[0051] In the embodiment of FIG. 2, the primary amplitude 46 ismonitored by the excitation control circuit 26 via a resistor dividernetwork, shown symbolically in FIG. 2 by the resistors R_(x) and R_(y).In the excitation control circuit 26, the monitored primary amplitude isrectified by a half wave rectifier 56 and then filtered by a low passfilter 58. A comparator 60 then compares the rectified and filteredmonitored primary amplitude to the reference parameter 27 to generate anexcitation control signal 62 that controls the excitation circuit 24.For example, in one aspect, the comparator 60 is a difference amplifier,the output of which varies continuously in proportion with a differencebetween its inputs (e.g., the rectified and filtered monitored primaryamplitude and the reference parameter 27).

[0052] As shown in FIG. 2, in one embodiment, the excitation circuit 24includes a fixed frequency clock 64 that serves as a frequency referencefor a pulse width modulator 66. The pulse width modulator 66 receivesthe excitation control signal 62 output by the excitation controlcircuit 26 and outputs a power driver control signal 68 to a powerdriver 70. The power driver 70 provides power from the power supply 20to the primary resonant circuit (formed by one or more capacitors 30 andthe primary winding 32) in the form of the pulses 50, which periodicallyinject energy into the primary resonant circuit.

[0053] In FIG. 2, the pulse width modulator 66 controls the width 52 ofthe pulses 50 based on the excitation control signal 62. In particular,according to one aspect of this embodiment, a wider pulse width 52supplies greater power to the primary resonant circuit, therebyincreasing the primary amplitude 46, whereas a shorter pulse width 52provides less power to the primary resonant circuit, thereby decreasingthe primary amplitude 46. In this manner, the power provided to theprimary resonant circuit is controlled based on a comparison of theprimary amplitude 46 and the reference parameter 27.

[0054]FIGS. 3A, 3B, and 3C are graphs that each show plots of thevoltage across the primary winding 32 as a finction of time (respectivetop plots), and a current 74 through the load 44 as a function of time(respective bottom plots) during operation of the energy transfer systemshown in FIG. 2. In the graph of FIG. 3A, it can be seen from the bottomplot that the load 44 coupled to the secondary winding 34 may vary, asillustrated by the variations in the load current 74; accordingly, theload 44 may have different power requirements from time to time.Generally, changes in the variable load 44 (i.e., varying powerrequirements) are observable as changes in a secondary voltage acrossthe secondary winding 34. Such changes in the secondary voltage in turnmay be observed as changes in the primary amplitude 46 of the primaryvoltage, as illustrated in the graphs of FIGS. 3A, 3B, and 3C, by virtueof the magnetic coupling between the primary winding 32 and thesecondary winding 34.

[0055] The graph of FIG. 3B is a horizontally expanded view (smallertime scale per horizontal division) of the region 76 in the graph ofFIG. 3A. In the region 76, the load current 74 is at a minimum (i.e.,the load requires relatively less power). Similarly, the graph of FIG.3C is a horizontally expanded view of the region 78 in the graph of FIG.3A, at which point the load current 74 is at a maximum (i.e., the loadrequires relatively more power).

[0056] As illustrated in the graph of FIG. 3B, when the load requiresrelatively less power, the excitation control circuit 26 monitors (i.e.,senses) an initial increase in the primary amplitude 46 due to the“lighter” load, and in response controls the excitation circuit 24 suchthat the pulse width 52 of the pulses 50 provided to the primaryresonant circuit (which can be viewed as artifacts in the sinusoidalwaveform in the graph of FIG. 3B) is decreased, resulting in acorresponding decrease in the primary amplitude 46. In contrast, asillustrated in the graph of FIG. 3C, when the load requires relativelymore power, the excitation control circuit 26 monitors an initialdecrease in the primary amplitude 46 due to the “heavier” load, and inresponse controls the excitation circuit 24 such that the pulse width 52is increased, resulting in a corresponding increase in the primaryamplitude 46. In both cases, the primary amplitude 46 is regulated basedon the reference parameter 27 to essentially a constant value, withnominal variations from this value due to a response time of thecircuits constituting the primary side feedback control loop (e.g., theexcitation circuit 24 and the excitation control circuit 26).

[0057] As discussed above, the energy transfer systems shown in FIGS. 1and 2 regulate the primary amplitude 46 to provide a sufficiently stableoutput power to the load 44 essentially independently of systemconditions associated with the secondary side of the energy transfersystem. In particular, in one aspect of this embodiment, regulation ofthe primary amplitude does not necessarily depend upon changes in theload and/or changes in relative position of the windings. Instead, byselecting an appropriate reference parameter 27, the primary amplitude46 is regulated such that a sufficiently stable power is provided to theload over a predetermined range for the load voltage 45, which mayfluctuate due to changes in the load and/or changes in the relativeposition of the primary and secondary windings.

[0058] In particular, in one aspect of the embodiment of FIGS. 1 and 2,a suitable desired range for the load voltage 45 is empiricallydetermined based on expected operating characteristics and powerrequirements of the load and particular design parameters of the energytransfer system. In this aspect, the desired range for the load voltage45 generally takes into consideration nominal changes in the load (i.e.,different power requirements from time to time) and changes in therelative position of the windings that may occur from time to timeduring normal operation of the system in a particular application. Thereference parameter 27 for the primary amplitude 46 is selected based onthe desired range for the load voltage 45, such that the load voltagefalls essentially within the desired range when the primary amplitude isregulated to a value that is proportional to the reference parameter.

[0059] Although the load voltage 45 in the embodiment of FIGS. 1 and 2may be susceptible to minor fluctuations due to changes in the loadand/or relative position of the windings notwithstanding the regulationof the primary amplitude, it should be appreciated that this embodimentnonetheless provides an acceptable solution for a variety of energytransfer system control applications with appropriate selection of thereference parameter 27. In particular, it should be appreciated that atleast one favorable feature of this embodiment is simplicity ofimplementation while nonetheless providing sufficient control of theenergy transfer system.

[0060]FIG. 3D is a circuit diagram showing one example of an electroniccircuit implementation of a primary side of the energy transfer systemshown in FIG. 2, according to one embodiment of the invention. In FIG.3D, the power source 20 is shown as 24 Volts supplied to a terminal ofone of two power transistors Q2 and Q3 that constitute a portion of thepower driver 70. The 24 Volt power source may be supplied, for example,by a suitable battery.

[0061] In one aspect of the embodiment of FIG. 3D, a target amplitudefor the primary amplitude 46 is in a range of from approximately 100Volts peak-to-peak to approximately 200 Volts peak-to-peak. In thisaspect, the capacitor 30 of the primary resonant circuit 48 shown inFIGS. 1 and 2 is implemented by capacitors C4, C7, C9, and C23. TheMOSFET drivers Q2 and Q3 switch between the power source 20 and a groundpotential, resulting in a 24 Volt peak-to-peak essentially rectangularwave drive signal which is amplified by the primary resonant circuit 48.The gates of the power transistors Q2 and Q3 are driven by integratedcircuit U3. In this embodiment, the primary amplitude 46 is regulated byadjusting a pulse width of the signals that are applied to the gates ofthe power transistors Q2 and Q3, as discussed in greater detail furtherbelow.

[0062] In the circuit shown in FIG. 3D, an exemplary frequency sourcefor the fixed frequency clock circuit 64 is shown as a 10 MHz crystaloscillator, which is divided down by counter U9 to approximately 312.5kHz. Alternatively, as discussed below in connection with anotherembodiment shown in FIG. 10, an adjustable RC oscillator may be employedto accommodate various resonant frequencies of the primary resonantcircuit 48. Timer U8A reduces a square wave output from U9 to a narrowtrigger pulse. which is output to U1 of the pulse width modulator 66.

[0063] In FIG. 3D, the primary amplitude 46 is sensed via a resistordivider network formed by resistor R6 in series with resistor R11 andvariable resistor R19. The sensed signal is half-wave rectified by diodeDS, filtered by resistor R10 and capacitor C19, and buffered by unitygain amplifier U4A. Amplifier U4B forms part of the comparator 60 (e.g.,a difference amplifier) to provide the excitation control signal 62. Theamplifier U4B compares the rectified and filtered monitored primaryamplitude signal to fixed reference 27, which in this example is a 2.5Volt reference provided by the reference source circuit 28, includingZener diode D6.

[0064] An error signal output by the amplifier U4B in FIG. 3D is limitedby diode D9 and resistors R18 and R12 and buffered by amplifier U4C toproduce the control signal 62. According to one aspect, the error signalis limited by the diode and resistors so as to limit the pulse width 52of the signal 68 output from the pulse width modulator 66 to a maximumof 50% duty cycle. This is done because maximum power transfer to theprimary resonant circuit occurs at 50% duty cycle. If the pulse widthwere allowed to increase beyond 50% duty cycle, the primary amplitudewould then be allowed to decrease as the error signal increased,effectively delivering less power to the primary resonant circuit whenthe feedback loop was trying to command more power, so as to increasethe primary amplitude.

[0065] The control signal 62 in FIG. 3D is applied to a voltagecontrolled timer U1 which forms part of the pulse width modulator 66. Inthe embodiment of FIG. 3D, if the rectified and filtered monitoredprimary amplitude input to U4B of the comparator 60 is lower than thereference voltage 27, the control signal 62 increases, causing a widthof pulses in the output signal 68 of the timer U1 to increase.Conversely, if the rectified and filtered monitored primary amplitudeinput to U4B of the comparator 60 is higher than the reference voltage27, the control signal 62 decreases, causing a width of pulses in theoutput signal 68 of the timer U1 to decrease. The decreased pulse widthin turn decreases the primary amplitude 46.

[0066]FIG. 4 is a block diagram similar to that of FIG. 1, showing anexample of an energy transfer system according to another embodiment ofthe invention. In the energy transfer system of FIG. 4, the fixedreference source 28 in the primary circuit 22 of FIGS. 1 and 2 isreplaced by a reference control circuit 31, which outputs a varyingreference parameter 29 based on a measurable characteristic of a signalassociated with the primary winding 32 (e.g., the primary amplitude 46).In this manner, the embodiment of FIG. 4 differs from that of FIGS. 1and 2, in that the primary amplitude 46 may be varied in a controlledmanner rather than regulated to an essentially constant value.Additionally, in the embodiment of FIG. 4, the measurable characteristicon which the varying reference parameter 29 is based may include someindication of changes in the load and/or changes in relative position ofthe primary and secondary windings. Accordingly, in one aspect, theembodiment of FIG. 4 actively controls (e.g., varies) the primaryamplitude 46 in response to changes in the load and/or relative windingposition.

[0067] In one aspect of the embodiment of FIG. 4, the magnetic field 36that couples the primary and secondary windings is viewed as forming aportion of a “power channel” 72, over which power is transferred fromthe power supply 20 to the variable load 44 at the particular frequency54, as illustrated in FIGS. 1 and 2 (e.g., approximately the resonantfrequency of the primary and secondary resonant circuits). Withreference again for the moment to FIG. 1, while power typically istransferred in a direction illustrated symbolically by the arrows of themagnetic field 36, Applicants have appreciated that information relatedto the secondary side of the energy transfer system, includinginformation related to changes in the variable load 44 and/or changes inrelative winding position, may be communicated in an opposite direction(i.e., from the secondary winding to the primary winding) via the powerchannel 72.

[0068] In view of the foregoing, according to one aspect of the energytransfer system shown in FIG. 4, the secondary circuit 42 of thesecondary side of the energy transfer system generates one or moreindications 76 (shown symbolically as an asterisk in FIG. 4) on thepower channel 72 that are detectable on the primary side of the energytransfer system. One or more detectable indications 76 provideinformation to the primary circuit 22 that is related, for example, tothe variable load 44 and the relative position of the windings. In oneaspect of this embodiment, the reference control circuit 31 includescircuitry to detect a detectable indication 76 on the power channel 72,as discussed further below in connection with FIGS. 5 and 10, and inturn outputs the varying reference parameter 29 based on one or moredetectable indications 76.

[0069] According to another aspect of this embodiment, as discussedfurther below in connection with FIG. 5, the secondary circuit 42 ofFIG. 4 provides one or more detectable indications 76 on the powerchannel 72 when the load voltage 45 (V_(load)) across the variable load44 exceeds a predetermined threshold load voltage. In response to one ormore detectable indications 76, the primary circuit 22 regulates theprimary amplitude 46 such that the load voltage 45 approximates thepredetermined threshold load voltage, notwithstanding changes in thevariable load and/or changes in the relative position of the windings.

[0070]FIG. 5 is a more detailed block diagram of the energy transfersystem shown in FIG. 4, according to one embodiment of the invention. Inone aspect of the embodiment of FIG. 5, detectable indications 76 on thepower channel 72 include one or more surges that are observable in theprimary amplitude 46 of the primary voltage across the primary winding32. FIG. 6 shows an example of detectable indications 76 on the powerchannel 72, in the form of surges that are observable in the primaryamplitude 46. It should be appreciated, however, that the invention isnot limited to the example of a detectable indication shown in FIG. 6,and that other types of detectable indications, as well as methods andapparatus for generating and detecting such indications, may beimplemented according to other embodiments of the invention.

[0071] In the embodiment of FIG. 5, the secondary circuit 42 includes acomparator 90 to make a comparison of the load voltage 45 across thevariable load 44 and a predetermined threshold voltage 88 (V_(thresh)).An output of the comparator 90 is coupled to a surge generator 78 (shownsymbolically in FIG. 5 as a switch) which generates one or more surgeson the power channel 72, based on the comparison of the load voltage 45and the predetermined threshold load voltage 88.

[0072] In one aspect of the embodiment of FIG. 5, the surge generator 78may include a shunt circuit, for example, in the form of a switch, thatdiverts the output power provided by the secondary winding 34 away fromthe variable load 44 when an input signal 86 to the shunt circuit(output by the comparator 90) reaches a shunt activation level. In oneembodiment, the comparator 90 generates the input signal 86 having theshunt activation level when the load voltage 45 exceeds thepredetermined threshold load voltage 88.

[0073] In FIG. 5, as the shunt circuit is activated, the combination ofthe capacitive load due to one or more capacitors 40 and the resistiveload due to the load 44 is replaced by the capacitive load due to theone or more capacitors 40 in series with the self-inductance of thesecondary winding 34. The activation of the shunt circuit creates ashort circuit across the resonant circuit formed by the one or morecapacitors 40 and the secondary winding 34. This, in turn, results in asurge in the current in the secondary winding. The surge thus generatedpropagates via the power channel 72 and is observable in the primaryvoltage.

[0074] According to one aspect of the energy transfer system of FIG. 5,the primary circuit 22 (comprising the excitation circuit 24, theexcitation control circuit 26, and the reference control circuit 31)regulates the primary amplitude 46 such that the input signal 86 to thesurge generator 78 (e.g., the shunt circuit) approximates andsubstantially remains below the shunt activation level (e.g, the inputsignal 86′ may be slightly above the shunt activation level for arelatively short time, but for the most part the input signal 86 remainsbelow the shunt activation level). Stated differently, the primaryamplitude 46 is regulated such that the shunt circuit has a relativelysmall activation duty cycle, so as to reduce any output power that maybe dissipated through the shunt circuit rather than provided to thevariable load. In some applications, a diversion of output power throughthe shunt circuit for a significant time period may produce undesirableheating in proximity to the variable load 44 (e.g., in applicationswhere the variable load 44 may be an implanted prosthesis in a human).Accordingly, in one aspect, the energy transfer system of FIG. 5 reducessuch undesirable heating by regulating the primary amplitude 46 suchthat the shunt circuit is activated for a relatively small portion ofthe time between successive activations (i.e., a relatively smallactivation duty cycle).

[0075] For purposes of the present discussion, in one aspect the terms“relatively small activation duty cycle” represent a time duration foractivation of the shunt circuit that is insufficient to achieveregulation of the load voltage via operation of the secondary circuit 42alone. In another aspect of the embodiment of FIG. 5, the primaryamplitude 46 is regulated such that the secondary circuit has anactivation time of less than approximately 100 microseconds, andconsecutive activations of the secondary circuit occur approximatelyevery 2 to 3 milliseconds. Alternatively, in yet another aspect, theprimary amplitude 46 is regulated such that the surge generator 78(e.g., the shunt circuit) has a duty cycle of less than approximately 1%(i.e., the shunt circuit is activated for less than approximately 1% ofthe time between consecutive activations of the shunt circuit). In yetanother aspect, the primary amplitude 46 is regulated such that theshunt circuit has a duty cycle of less than approximately 0.5%. In yetanother aspect, the primary amplitude 46 is regulated such that theshunt circuit has a duty cycle of less than approximately 0.1%, so as toreduce power dissipation through the shunt circuit.

[0076] To monitor and respond to a detectable indication 76 on the powerchannel 72 provided by the secondary circuit 42, FIG. 5 shows that thereference control circuit 31 includes a detector circuit 80, coupled toan output of the low pass filter 58 of the excitation control circuit26, to detect the detectable indication and to provide a detectedindication signal 81. In the embodiment of FIG. 5, the reference controlcircuit 31 also includes a timer 82 coupled to the detector circuit 80to receive the detected indication signal 81. Additionally, thereference control circuit 31 of FIG. 5 includes an output circuit 84coupled to the timer to output the varying reference parameter 29 basedat least on the detected indication signal 81. It should be appreciated,however, that the reference control circuit 31 is not limited to theimplementation shown in FIG. 5; in particular, in another embodimentdiscussed below in connection with FIG. 11, the timer 82 and the outputcircuit 84 may be replaced by alternative circuitry.

[0077] With reference again to FIG. 5, the output circuit 84 in thisembodiment is shown as an integrator having a time constant based oncapacitor 91 and resistor 93. In one aspect, the timer 82 controls theintegrator such that the integrator outputs the varying referenceparameter as a varying reference voltage having an essentiallytriangular waveform. In another aspect, values for the capacitor 91 andthe resistor 93, as well as values for components associated with thetimer 80 (as illustrated for example, in FIG. 10) are chosen such thatthe varying reference parameter 29 has a frequency in a range of fromapproximately 5 Hz to approximately 40 Hz, as discussed further below.

[0078] According to one embodiment of the invention, the referencecontrol circuit 31 shown in FIG. 5 fumctions as follows. The detector 80monitors the rectified and filtered primary amplitude for one or moredetectable indications 76 (e.g., one or more surges), and if adetectable indication is detected, the detector outputs a detectedindication signal 81 which triggers the timer 82. The timer 82 outputs atimer output signal 94 to a first input of the output circuit 84, whichsignal switches between a high level and a low level. In particular,once the timer is triggered by the detected indication signal 81, thetimer output signal 94 goes to the high level for a fixed time. Thetimer 82 may be triggered one or more times by the detected indicationsignal 81, and the timer output signal 94 remains at the high level ifthe detectable indications continue to be detected.

[0079] In FIG. 5, a second input of the output circuit 84 receives areference voltage 92 (labeled as V_(ref) in FIG. 5), which voltage isbetween the high and low levels of the timer output signal 94. Forexample, in one aspect of this embodiment, the voltage V_(ref) is set toapproximately midway between the high and low timer output levels. Whilethe timer output signal 94 is at the high level (i.e., above thereference voltage 92), the varying reference parameter 29 (e.g., avarying voltage in this example) output by the output circuit 84 rampsdownward at a rate that is related to the respective values of thecapacitor 91 and the resistor 93. Since the varying reference parameter29 ramps downward, the primary amplitude 46 ramps downward as well totrack the changes in the varying reference parameter. When the primaryamplitude 46 is reduced such that the load voltage 45 is less than thethreshold voltage 88 in the secondary circuit 42, the secondary circuit42 stops generating the detectable indications 76 and, accordingly, thedetector 80 stops triggering the timer 82.

[0080] The integrating output circuit 84 of FIG. 5 continues rampingdown until the timer 82 resets the timer output signal 94 to the lowlevel. The timer 82 may be implemented so as to have a variety ofpossible reset times and duty cycles, as discussed further below inconnection with FIGS. 7 and 10. Once the timer resets the timer outputsignal 94 to the low level, the signal 94 is below the reference voltage92; hence, the integrating output circuit 84 begins ramping up thevarying reference parameter 29 (at a rate that is related to therespective values of the capacitor 91 and the resistor 93). Accordingly,it should be appreciated that one or more of the reference voltage 92,the component values of the capacitor 91 and the resistor 93, and theduty cycle of the timer 82, may be selected to implement varioustimings/waveforms for the varying reference parameter 29. For example,as discussed above, the reference voltage 92 may be selected to behalf-way between the high and low levels of the timer output signal 94,so as to implement an essentially triangular waveform for the varyingreference parameter 29. It should also be appreciated, however, that theinvention is not limited in this respect, as other analog and/or digitalcircuits may be implemented to output other waveforms for the varyingreference parameter 29, as discussed further below.

[0081] In the embodiment of FIG. 5, as the varying reference parameter29 ramps upward, the primary amplitude 46 increases to track theincreasing varying reference parameter, until the load voltage 45approaches and approximates the threshold voltage 88 in the secondarycircuit 42. At this point, the secondary circuit 42 generates anotherdetectable indication 76, and the foregoing process is repeated.Accordingly, the system of FIG. 5 is capable of regulating the loadvoltage 45 in response to changes in the load and/or changes in relativeposition of the primary and secondary windings because regardless ofeither or both of these changes, the primary circuit 22 continuouslyseeks to adjust the primary amplitude 46 based on the detectableindications 76 (i.e., such that the load voltage 45 approaches andapproximates the threshold voltage 88 in the secondary circuit 42).

[0082]FIG. 7 is a graph showing plots of examples of various signals inthe energy transfer system of FIG. 5 during the above-describedregulation process, according to one embodiment of the invention. Forexample, the top-most plot in FIG. 7 illustrates the detected indicationsignal 81, the next lower plot illustrates the timer output signal 94,the next lower plot illustrates the varying reference parameter 29(which in one example is a varying voltage), and the bottom-most plotillustrates the load voltage 45 (V_(load)) that results from theabove-described regulation process.

[0083] With reference to FIG. 7, according to one embodiment, afrequency of the timer output signal 94 (i.e., the inverse of a period96 of the timer output signal) and, hence, a fundamental frequency ofthe varying reference parameter 29, generally is selected to be fastenough so that the primary circuit 22 may effectively respond to changesin the load and/or changes in the relative position of the windings,which changes may require significant changes to the primary amplitude46. However, in one aspect, the frequency of the timer output signal 94is selected to additionally take into account that higher activationfrequencies of the surge generator 78 of the secondary circuit 42 mayproduce undesirable power dissipation and excessive heating in someapplications. Accordingly, at least two relevant criteria (i.e.,practical response time given a particular application and powerdissipation in the secondary side of the energy transfer system) may beconsidered in selecting an appropriate frequency for the timer outputsignal 94. In some instances, the frequency of the timer output signal94 may be determined empirically, depending on the particularapplication of the energy transfer system. For example, in oneembodiment, the frequency of the timer output signal 94 is selected tobe in a range of from approximately 5 Hz to approximately 40 Hz.

[0084] With reference again to FIG. 5, in yet another embodiment, thevarying reference parameter 29 is reduced immediately followingdetection of a detectable indication rather than ramping down, as isaccomplished by the integrating output circuit 84 shown in FIG. 5. Tothis end, the output circuit 84 may be configured to output a saw-toothwaveform for the varying reference parameter (i.e., rapid fall, slowrise). In one aspect of this embodiment, the output circuit 84 may beimplemented as a micro-controller that is configured so as to output asaw-tooth waveform based on one or more detectable indications, asopposed to the essentially triangular waveform output by the example ofthe integrating output circuit shown in FIG. 5. In particular, byimplementing such a micro-controller, the primary circuit can rapidlyreduce the primary amplitude 46 and then gradually increase it at a muchslower rate. At least one advantage of implementing a micro-controllerso as to provide a saw-tooth waveform varying reference parameter 29 isto safeguard against rapid variations in the load 44 which canpotentially lead to harmful voltage spikes.

[0085] Yet another variation of this scheme is to initiate a timevarying, adaptive rate for the increase or decrease of the primaryamplitude 46. For example, upon detecting one or more indications 76 onthe primary side (i.e., the detector 80 of the reference control circuit31 provides the detected indication signal. 81), each successiveindication in a series of indications could be made to correspond to alarger step reduction in primary amplitude, and, as the indicationsstop, the rate of increase of the primary amplitude 46 could be made toincrease as time passes. This scheme allows for fine adjustment of theprimary amplitude around any given operating point, as well as the rapidconvergence of the primary amplitude to a different operating point incase of an abrupt change of the load 44 or an abrupt change in relativewinding position.

[0086] In another embodiment of the energy transfer system shown in FIG.5, an auxiliary output of the detector 80 is connected to an auxiliaryinput of the power driver 70. In this manner, the detector 80 mayprovide a squelch signal 87 to the power driver 70 so as to reduce asurge current in the primary winding 32 upon the detection of one ormore detectable indications 76 (e.g., surges) on the power channel 72.Accordingly, the squelch signal 87, along with the excitation controlsignal 62 provided by the excitation control circuit 26, may be employedto further regulate the primary amplitude 46.

[0087]FIG. 8A and 8B are graphs illustrating the operation of thesquelch signal 87. Each of the graphs in FIGS. 8A and 8B shows the loadvoltage 45 (respective top plots) and a current 102 through the primarywinding 32 (respective bottom plots) during a time period indicated bythe reference character 100 shown in FIG. 7, namely, near and during thegeneration of a detectable indication or surge on the power channel. Thegraph of FIG. 8A represents the load voltage and the current through theprimary winding if the squelch signal 87 is not used, whereas the graphof FIG. 8B represents the load voltage and the current through theprimary winding if the squelch signal 87 is used, as illustrated in FIG.5. From a comparison of the graphs in FIGS. 8A and 8B, it may beappreciated that employing the squelch signal 87 facilitates a reductionin a surge current in the primary winding during a detectableindication, due to the fact that, in this embodiment, the detector 80responds more quickly to detectable indications on the power channelthan does the excitation control circuit 26. It should be appreciated,however, that the energy transfer system of FIG. 5 may effectivelyregulate power with or without the implementation of the squelch signal87.

[0088]FIG. 9 is a circuit diagram showing one example of an electroniccircuit implementation of a secondary side of the energy transfer systemshown in FIG. 5, according to one embodiment of the invention. In theexemplary circuit of FIG. 9, a self-inductance of the secondary winding34 is approximately 30 μH. A mutual inductance between the primarywinding 32 and the secondary winding 34 is approximately 0.4 μH.Consequently, since the self-inductance in this example is significantlylarger than the mutual inductance of the windings, most of the secondarywinding self-inductance appears in series with the load 44. CapacitorsC1 and C4, which constitute the secondary capacitor 40 shown in otherfigures, are selected such that a resonance frequency of the secondarycircuitry is approximately 300 kHz, at which frequency the secondarywinding has an inductive reactance of approximately 50 ohms. Thecapacitors C1 and C4 reduce the reactance to a few ohms.

[0089] In the secondary circuit 42 shown in the example of FIG. 9,diodes D1-D4 form a full wave bridge rectifier, and a capacitor networkformed by the capacitors C2, C3, C6 and C7 acts to smooth the rectifieroutput voltage. Resistors R9 and R10 function to balance a DC voltageacross the capacitor network. Transistors Q1 and Q2 at the input of thebridge rectifier constitute the surge generator 78, and are turned on toshunt the secondary voltage. Comparator 90 (U1). senses the load voltage45 through a resistor divider formed by R1, R2, and R5. When a voltageat an output of this divider exceeds the threshold voltage 88 applied tothe inverting input of the comparator 90, which in this example is setat approximately 2.5 Volts, the signal 86 goes high to drive thetransistors Q1 and Q2. The comparator output that ultimately generatesthe signal 86 is buffered by transistors Q3 and Q4. Resistors R6 and R7dampen oscillations that may occur when the gate terminals oftransistors Q1 and Q2 are driven in parallel by the signal 86. Zenerdiode D7 limits the gate voltage of transistors Q3 and Q4 to protect thetransistors Q1 and Q4. Resistor R5 provides positive feedback to thecomparator 90 which establishes a reset level for the over-voltagedetection that is several volts lower than the activation thresholdlevel (i.e., a hysteresis band) to reduce noise sensitivity and preventhigh frequency oscillation.

[0090] Using the component values shown in the exemplary circuit of FIG.9 (i.e., R1=54.9 Kohms, R2=4.99 Kohms, and R5=49.9 Kohms), the signal 86goes high to activate the transistors Q1 and Q2 when the load voltage 45is approximately 33 Volts, and goes low to deactivate the transistors Q1and Q2 when the load voltage 45 is approximately 19 Volts. However, itshould be appreciated that the invention is not limited to theparticular arrangement of components and the particular component valuesillustrated in FIG. 9, and that other circuit configurations arepossible to implement the various fimctions of the secondary circuit 42discussed herein, according to various embodiments of the invention. Forexample, in one embodiment, the various components discussed above inFIG. 9 may be selected such that the signal 86 goes high to activate thetransistors Q1 and Q2 when the load voltage 45 is in a range of fromapproximately 30 Volts to approximately 45 Volts, and such that thesignal 86 goes low to deactivate the transistors Q1 and Q2 when the loadvoltage 45 is in a range of from approximately 20 Volts to approximately30 Volts.

[0091]FIG. 10 is a circuit diagram showing one example of an electroniccircuit implementation of both the primary and secondary sides of theenergy transfer system shown in FIG. 5, according to one embodiment ofthe invention. The secondary circuit 42 shown in FIG. 10 issubstantially similar to that shown in FIG. 9. Similarly, the primarycircuit 22 shown in FIG. 10 is substantially similar to that shown inFIG. 3D, albeit with the addition of components related to the variablereference control circuit 31.

[0092] In the exemplary circuit of FIG. 10, comparator U100 of thereference control circuit 31 serves as a portion of the surge detector80, which receives as an input the rectified and filtered monitoredprimary amplitude signal and a variable surge detector threshold signalthat tracks a short-term average of the monitored primary amplitudesignal, with an offset of one diode forward voltage drop to provide somenoise immunity. Without the offset to the short-term average voltageused as the reference for the comparator 100, both inputs of thecomparator 100 dwell at essentially the same voltage, making it verysensitive to input noise. An output of the comparator U100 provides thedetected indication signal 81 as well as the squelch signal 87. Thedetected indication signal 81 triggers the integrated circuit U28, whichserves as the timer 82. The resistive and capacitive componentsassociated with the timer U28 are selected to effect a desired frequencyfor the variable reference parameter 29. In the particular example shownin FIG. 10, these components are selected such that the timer has a timeconstant of approximately 17.5 milliseconds, which ultimately results ina frequency of approximately 28 Hz for the timer output signal 94 (and,hence, the variable reference parameter 29). It should be appreciated,however, that the invention is not limited in this respect, as othertime constants and resulting frequencies may be suitable in otherembodiments of the invention. For example, as discussed above inconnection with FIGS. 5 and 7, in one embodiment the frequency of thetimer output signal 94 is selected to be in a range of fromapproximately 5 Hz to approximately 40 Hz.

[0093] To generate the variable reference parameter 29, the primarycircuit 22 shown in FIG. 10 also includes the integrator output circuit84. The inverting input of this integrator receives the timer outputsignal 94. In the exemplary circuit of FIG. 10, the timer output signal94 switches between 0 Volts and 5 Volts, and the non-inverting input ofthe integrator is biased at a reference voltage 92 of 2.5 Volts. Underthese conditions, the reference parameter 29 is in equilibrium when thetimer output signal 94 spends equal time in the high and low states.

[0094] As discussed above in connection with FIG. 5, in the circuit ofFIG. 10 if the detector 80 detects one or more indications, the timer 82is triggered to go to a high state for time equal to the time constantof the timer. The timer is retriggerable and remains in the high stateif indications continue to be detected. While the timer output signal isin a high state, the reference parameter 29 ramps downward at a raterelated to the component values of the capacitor 91 and the resistor 93,thereby decreasing the primary amplitude 46. As the primary amplitudedecreases, eventually the secondary circuit 42 stops generatingdetectable indications, and the detector 80 stops triggering the timer82. The reference parameter 29 continues ramping down until the timeroutput signal resets to a low level, after which the reference parameter29 begins ramping up again (at a rate related to the component values ofthe capacitor 91 and the resistor 93) until indications generated by thesecondary circuit are again detected.

[0095]FIG. 11 is a circuit diagram showing another example of anelectronic circuit implementation of the primary side of the energytransfer system of FIG. 5, according to one embodiment of the invention.In particular, in the circuit of FIG. 11, an implementation alternativeto that shown in FIG. 10 is given for the reference control circuit 31.More specifically, with reference again to FIG. 5, the detector 80, thetimer 82, and the output circuit 84 of the reference control circuit 31are implemented in FIG. 11 using the operational amplifier U7D andseveral other circuit components associated therewith.

[0096] In the circuit of FIG. 11, the non-inverting input of theoperational amplifer U7A receives a rectified and filtered monitoredprimary amplitude signal. The inverting input of operational amplifierU7A receives a low-pass filtered version of this signal (as a result ofthe resistor RX2 and the capacitor C121), with a small bias added viaresistor R120. The output of U7A is received by the surge detector 80(operational amplifier U7D), which provides the detected indicationsignal 81.

[0097] In FIG. 11, the detected indication signal 81 is used to activatetransistor Q3 (labeled with reference character 83B) which, whenactivated, provides a discharge path for capacitor C23 through resistorRX2. The capacitor C23, together with resistors R19 and R20(collectively labeled with reference character 83A in FIG. 11) providethe varying reference voltage 29 to the comparator 60 (operationalamplifier U7B). 30. Accordingly, the circuit components collectivelydesignated by 83A and 831B in FIG. 11 provide alternative circuitimplementations for the timer 82 and the output circuit 84 shown inFIGS. 5 and 10. In particular, detectable indications detected by thesurge detector 80 cause the detected indication signal 81 to activatetransistor 83B (Q3) for a short time duration,, which begins todischarge the capacitor C23, thereby reducing the varying referencevoltage 29 at the non-inverting input of U7B. When the transistor 83B isnot activated (i.e., in the absence of detected indications), thecapacitor C23 is allowed to charge through the resistor R19, and thevarying reference voltage 29 increases accordingly. Hence, the varyingreference voltage 29 is allowed to increase until a detectableindication is detected, which then activates the transistor 83B, beginsto discharge capacitor C23, and decreases the reference voltage 29. Itshould be appreciated that, in this embodiment, the waveform of thereference voltage 29 is not necessarily a triangular waveform, butrather has an exponential shape representative of a charging anddischarging of an RC circuit.

[0098] It should be appreciated that the foregoing discussion of theexemplary circuits of FIGS. 9-11 is for purposes of illustration only,and that the invention is not limited to these specific implementations.

[0099] Having described several embodiments of the invention in detail,various modifications and improvements will readily occur to thoseskilled in the art. Such modifications and improvements are intended tobe within the spirit and scope of the invention. Accordingly, theforegoing description is by way of example only, and is not intended aslimiting. The invention is limited only as defined by the followingclaims and the equivalents thereto.

What is claimed is:
 1. An apparatus for use in a transcutaneous energytransfer system that includes a power supply, a primary winding and asecondary winding, the apparatus comprising: a control circuit, coupledto the power supply and the primary winding, to monitor a primaryamplitude of a primary voltage across the primary winding and to controlthe primary amplitude based only on the monitored primary amplitude anda reference voltage.
 2. The apparatus of claim 1, in combination withthe primary winding.
 3. The combination of claim 2, further includingthe secondary winding.
 4. The apparatus of claim 1, wherein the controlcircuit regulates the primary amplitude such that the primary voltagehas essentially a constant frequency.
 5. The apparatus of claim 1,wherein the reference voltage is fixed during normal operation of theenergy transfer system.
 6. The apparatus of claim 1, wherein the controlcircuit includes: at least one capacitor coupled to the primary windingto form a primary resonant circuit with the primary winding; anexcitation circuit coupled to the power supply and the primary resonantcircuit to provide power to the primary resonant circuit; and anexcitation control circuit coupled to the excitation circuit and theprimary winding to control the excitation circuit based only on themonitored primary amplitude and the reference voltage.
 7. The apparatusof claim 6, wherein: the excitation circuit outputs pulses to theprimary resonant circuit to provide the power to the primary resonantcircuit; and the excitation control circuit controls a width of thepulses output by the excitation circuit such that the primary amplitudeis approximately equal to a predetermined amplitude that is proportionalto the reference voltage.
 8. The apparatus of claim 7, in combinationwith the primary winding.
 9. The combination of claim 8, furtherincluding the secondary winding.
 10. The apparatus of claim 1, whereinthe reference voltage varies based on the primary amplitude duringnormal operation of the energy transfer system.
 11. The apparatus ofclaim 10, wherein the reference voltage varies based on surges in themonitored primary amplitude during the normal operation of the energytransfer system.
 12. The apparatus of claim 11, wherein the controlcircuit includes: a reference control circuit to control the varyingreference voltage based on the surges in the monitored primaryamplitude.
 13. The apparatus of claim 12, wherein the reference controlcircuit includes: a surge detector to detect the surges in the monitoredprimary amplitude; a timer circuit coupled to the surge detector; and anoutput circuit coupled to the timer circuit, wherein the timer circuitcontrols the output circuit to output the varying reference voltagebased on the detected surges in the monitored primary amplitude.
 14. Theapparatus of claim 13, wherein the timer circuit controls the outputcircuit such that the output circuit outputs the varying referencevoltage as an essentially triangular waveform.
 15. The apparatus ofclaim 13, wherein the timer circuit controls the output circuit suchthat the output circuit outputs the varying reference voltage as anessentially saw-tooth waveform.
 16. The apparatus of claim 13, whereinthe control circuit further includes: at least one capacitor coupled tothe primary winding to form a primary resonant circuit with the primarywinding; an excitation circuit coupled to at least the power supply andthe primary resonant circuit to provide power to the primary resonantcircuit; and an excitation control circuit, coupled to the excitationcircuit, the primary winding, and the reference control circuit, tocontrol the excitation circuit based only on the monitored primaryamplitude and the varying reference voltage.
 17. The apparatus of claim16, wherein: the excitation circuit outputs pulses to the primaryresonant circuit to provide the power to the primary resonant circuit;and the excitation control circuit controls a width of the pulses outputby the excitation circuit such that the primary amplitude isapproximately equal to a predetermined amplitude that is proportional tothe varying reference voltage.
 18. The apparatus of claim 16, whereinthe surge detector is coupled to the excitation circuit so as to controlthe power provided by the excitation circuit to the primary resonantcircuit based on at least the detected surges in the monitored primaryamplitude.
 19. The apparatus of claim 18, in combination with theprimary winding.
 20. The combination of claim 19, firther including thesecondary winding.
 21. In a transcutaneous energy transfer systemincluding a power supply, a primary winding and a secondary winding, amethod comprising acts of: monitoring a primary amplitude of a primaryvoltage across the primary winding; and regulating the primary amplitudebased only on the monitored primary amplitude and a reference voltage.22. The method of claim 21, wherein the act of regulating includes anact of maintaining an essentially constant frequency of the primaryvoltage.
 23. The method of claim 21, wherein the act of regulatingincludes an act of comparing a sample of the monitored primary amplitudeto a fixed reference voltage, the sample being proportional to themonitored primary amplitude.
 24. The method of claim 21, wherein theenergy transfer system further includes at least one capacitor coupledto the primary winding to form a primary resonant circuit with theprimary winding, and wherein the act of regulating the primary amplitudeincludes an act of controlling power provided to the primary resonantcircuit based only on the monitored primary amplitude and the referencevoltage.
 25. The method of claim 24, wherein the act of controllingpower includes an act of controlling a width of pulses provided to theprimary resonant circuit such that the primary amplitude isapproximately equal to a predetermined amplitude that is proportional tothe reference voltage.
 26. The method of claim 21, wherein the act ofregulating includes an act of making a comparison of a sample of themonitored primary amplitude and a varying reference voltage, the samplebeing proportional to the monitored primary amplitude.
 27. The method ofclaim 26, wherein the act of making a comparison includes an act ofgenerating the varying reference voltage based on surges in themonitored primary amplitude during normal operation of the energytransfer system.
 28. The method of claim 27, wherein the act ofgenerating the varying reference voltage includes an act of generatingthe varying reference voltage as an essentially triangular waveformbased on the surges in the monitored primary amplitude.
 29. The methodof claim 27, wherein the act of generating the varying reference voltageincludes an act of generating the varying reference voltage as anessentially triangular waveform based on the surges in the monitoredprimary amplitude.
 30. The method of claim 27, wherein the energytransfer system further includes at least one capacitor coupled to theprimary winding to form a primary resonant circuit with the primarywinding, and wherein the act of regulating the primary amplitude furtherincludes act of controlling power provided to the primary resonantcircuit based on at least the comparison.
 31. The method of claim 30,wherein the act of controlling power includes an act of controlling awidth of pulses provided to the primary resonant circuit such that theprimary amplitude is approximately equal to a predetermined amplitudethat is proportional to the varying reference voltage.
 32. The method ofclaim 30, wherein the act of controlling power includes an act ofcontrolling the power based on the comparison and the surges in themonitored primary amplitude.
 33. An energy transfer system fortransferting power from a power supply located on a first side of aphysical boundary to a variable load located on a second side of thephysical boundary, the energy transfer system comprising: a primarywinding electrically coupled to the power supply to generate a magneticfield based on input power provided by the power supply, the magneticfield permeating the physical boundary; a secondary winding,magnetically coupled to the primary winding via the magnetic field, toreceive at least a portion of the magnetic field, the secondary windingelectrically coupled to the variable load to provide output power to thevariable load based on the received magnetic field; and at least onecontrol circuit, electrically coupled to at least the primary winding,to regulate a primary voltage across the primary winding such that asufficiently stable output power is provided to the variable-loadnotwithstanding at least one of changes in the load and changes in arelative position of the primary winding and the secondary winding. 34.The system of claim 33, wherein the at least one control circuitregulates a load voltage across the variable load based on at least oneof the changes in the variable load and the changes in the relativeposition of the primary winding and the secondary winding.
 35. Thesystem of claim 33, wherein the at least one control circuit regulatesthe primary voltage based on at least one of the changes in the load andthe changes in the relative position of the primary winding and thesecondary winding.
 36. The system of claim 35, wherein: the power istransferred from the power supply to. the variable load via a powerchannel, wherein the magnetic field that couples the primary andsecondary windings forms a portion of the power channel; and the atleast one control circuit obtains information related to at least one ofthe changes in the variable load and the changes in the relativeposition of the primary winding and the secondary winding via the powerchannel.
 37. The system of claim 36, wherein the power channel hasessentially a constant frequency.
 38. The system of claim 33, whereinthe at least one control circuit includes: at least one capacitorcoupled to the primary winding to form a primary resonant circuit withthe primary winding; an excitation circuit coupled to at least the powersupply and the primary resonant circuit to provide the input power tothe primary resonant circuit; and an excitation control circuit coupledto the excitation circuit and the primary winding to control theexcitation circuit so as to regulate the primary voltage.
 39. The systemof claim 38, wherein the excitation control circuit controls theexcitation circuit based on a comparison of a reference voltage and aprimary amplitude of the primary voltage.
 40. The system of claim 39,wherein: the excitation circuit outputs pulses to the primary resonantcircuit to provide the input power to the primary resonant circuit; andthe excitation control circuit controls a width of the pulses output bythe excitation circuit such that the primary amplitude is approximatelyequal to a predetermined amplitude that is proportional to the referencevoltage.
 41. The system of claim 33, wherein the power is transferredfrom the power supply to the variable load via a power channel, whereinthe magnetic field that couples the primary and secondary windings formsa portion of the power channel, and wherein the at least one controlcircuit includes: a first control circuit electrically coupled to theprimary winding to regulate the primary voltage based on at least one ofthe changes in the variable load and the changes in the relativeposition of the primary winding and the secondary winding; and a secondcontrol circuit electrically coupled to the secondary winding to providea detectable indication on the power channel that indicates when a loadvoltage across the variable load exceeds a predetermined threshold loadvoltage.
 42. The system of claim 41, wherein the first control circuitregulates the primary voltage based on the detectable indication. 43.The system of claim 42, wherein the first control circuit regulates theprimary voltage such that the load voltage approximates thepredetermined threshold load voltage notwithstanding at least one of thechanges in the variable load and changes in the relative position of theprimary winding and the secondary winding.
 44. The system of claim 42,wherein the first control circuit includes a detector circuit coupled tothe primary winding to detect the detectable indication on the powerchannel.
 45. The system of claim 44, wherein the first control circuitincreases a primary amplitude of the primary voltage if the detectorcircuit does not detect the detectable indication and decreases theprimary amplitude if the detector circuit detects the detectableindication.
 46. The system of claim 44, wherein: the detectableindication includes at least one surge in the primary amplitude; and thedetector circuit includes a surge detector.
 47. The system of claim 46,wherein the second control circuit includes: a comparator to make acomparison of the load voltage to the predetermined threshold loadvoltage; and a surge generator coupled to the comparator to generate theat least one surge based on the comparison.
 48. The system of claim 47,wherein the surge generator includes a shunt circuit that diverts theoutput power provided by the secondary winding away from the variableload when an input signal to the shunt circuit reaches a shuntactivation level, the comparator providing the input signal having theshunt activation level when the load voltage exceeds the predeterminedthreshold load voltage.
 49. The system of claim 48, wherein the firstcontrol circuit regulates the primary voltage such that the input signalto the shunt circuit approximates and substantially remains below theshunt activation level.
 50. The system of claim 48, wherein the firstcontrol circuit regulates the primary voltage such that the shuntcircuit has a duty cycle of less than approximately 1%.
 51. The systemof claim 46, wherein the first control circuit regulates the primaryvoltage based on a varying reference voltage that is derived from thedetectable indication.
 52. The system of claim 51, wherein the firstcontrol circuit includes a reference control circuit to generate thevarying reference voltage, the reference control circuit including: thedetector circuit to detect the detectable indication on the powerchannel and to provide a detected indication signal; a timer circuitcoupled to the detector circuit to receive the detected indicationsignal; and an output circuit coupled to the timer circuit, wherein thetimer circuit controls the output circuit to output the varyingreference voltage based on the detected indication signal.
 53. Thesystem of claim 52, wherein the timer circuit controls the outputcircuit such that the output circuit outputs the varying referencevoltage as an essentially triangular waveform.
 54. The system of claim52, wherein the timer circuit controls the output circuit such that theoutput circuit outputs the varying reference voltage as an essentiallysaw-tooth waveform.
 55. The system of claim 52, wherein the varyingreference voltage has a frequency in a range of from approximately 5 Hzto approximately 40 Hz.
 56. The system of claim 52, wherein the firstcontrol circuit further includes: at least one capacitor coupled to theprimary winding to form a primary resonant circuit with the primarywinding; an excitation circuit coupled to at least the power supply andthe primary resonant circuit to provide the input power to the primaryresonant circuit; and an excitation control circuit, coupled to theexcitation circuit, the primary winding, and the reference controlcircuit, to control the excitation circuit based on the primary voltageand the varying reference voltage.
 57. The system of claim 56, wherein:the excitation circuit outputs pulses to the primary resonant circuit toprovide the input power to the primary resonant circuit; and theexcitation control circuit controls a width of the pulses output by theexcitation circuit such that a primary amplitude of the primary voltageis approximately equal to a predetermined amplitude that is proportionalto the varying reference voltage.
 58. The system of claim 57, whereinthe detector circuit is coupled to the excitation circuit so as tocontrol the input power provided by the excitation circuit to theprimary resonant circuit based on at least the detectable indication.59. The system of claim 58, wherein: the detectable indication includesat least one surge in the primary amplitude; and the detector circuitincludes a surge detector.
 60. The system of claim 59, wherein thesecond control circuit includes: a comparator to make a comparison ofthe load voltage to the predetermined threshold load voltage; and asurge generator coupled to the comparator to generate the at least onesurge based on the comparison.
 61. The system of claim 60, wherein thesurge generator includes a shunt circuit that diverts the output powerprovided by the secondary winding away from the variable load when aninput signal to the shunt circuit reaches a shunt activation level, thecomparator providing the input signal having the shunt activation levelwhen the load voltage exceeds the predetermined threshold load voltage.62. The system of claim 61, wherein the first control circuit regulatesthe primary voltage such that the input signal to the shunt circuitapproximates and substantially remains below the shunt activation level.63. The system of claim 61, wherein the first control circuit regulatesthe primarv voltage such that the shunt circuit has a duty cycle of lessthan approximately 1%.
 64. In an energy transfer system that includes aprimary winding electrically coupled to a power supply located on afirst side of a physical boundary, the energy transfer system furtherincluding a secondary winding electrically coupled to a variable loadlocated on a second side of the physical boundary, a method oftransferring power from the power supply to the variable load comprisingan act of: regulating a primary voltage across the primary winding so asto provide a sufficiently stable output power to the variable loadnotwithstanding at least one of changes in the variable load and changesin a relative position of the primary winding and the secondary winding.65. The method of claim 64, wherein the act of regulating a primaryvoltage includes an act of regulating the primary voltage based on atleast one of the changes in the variable load and the changes in therelative position of the primary winding and the secondary wmiding. 66.The method of claim 65, wherein the power is transferred from the powersupply to the variable load via a power channel, wherein a magneticfield that couples the primary and secondary windings forms a portion ofthe power channel, and wherein the act of regulating includes an act of:obtaining information related to at least one of the changes in thevariable load and the changes in the relative position of the primarywinding and the secondary winding via the power channel.
 67. The methodof claim 66, wherein the power channel has essentially a constantfrequency.
 68. The method of claim 64, wherein the act of regulatingincludes an act of regulating the primary voltage based on a comparisonof a reference voltage and the primary voltage.
 69. The method of claim68, wherein the energy system filrther includes at least one capacitorcoupled to the primary winding to form a primary resonant circuit, andwherein the act of regulating includes an act of: controlling a width ofpulses input to the primary resonant circuit so as to regulate the inputpower to the primary resonant circuit.
 70. The method of claim 69,wherein the act of controlling a width of pulses includes an act ofcontrolling the width of the pulses such that a primary amplitude of theprimary voltage is approximately equal to a predetermined amplitude thatis proportional to the reference voltage.
 71. The method of claim 64,wherein the power is transferred from the power supply to the variableload via a power channel, wherein a magnetic field that couples theprimary and secondary windings forms a portion of the power channel, andwherein the method furlther includes an act of: generating a detectableindication on the power channel that indicates when a load voltageacross the variable load exceeds a predetermined threshold load voltage.72. The method of claim 71, wherein the act of regulating includes anact of regulating the primary voltage based on the detectableindication.
 73. The method of claim 72, wherein the act of regulatingincludes an act of regulating the primary voltage such that the loadvoltage approximates the predetermined threshold load voltagenotwithstanding at least one of the changes in the variable load and thechanges in the relative position of the primary winding and thesecondary winding.
 74. The method of claim 72, further including an actof detecting the detectable indication on the power channel.
 75. Themethod of claim 74, wherein the act of regulating includes acts of:increasing a primary amplitude of the primary voltage if the detectorcircuit does not detect the detectable indication; and decreasing theprimary amplitude if the detector circuit detects the detectableindication.
 76. The method of claim 75, wherein: the act of generating adetectable indication includes an act of generating at least one surgein the primary amplitude; and the act of detecting the detectableindication includes an act of detecting the at least one surge in theprimary amplitude.
 77. The method of claim 76, wherein the act ofgenerating at least one surge includes acts of: making a comparison ofthe load voltage to the predetermined threshold load voltage; andgenerating the at least one surge based on the comparison.
 78. Themethod of claim 77, wherein the act of generating the at least one surgebased on the comparison includes acts of: diverting the output powerprovided by the secondary winding away from the variable load when anactivation signal is generated; and generating the activation signalwhen the load voltage exceeds the predetermined threshold load voltage.79. The method of claim 78, wherein the act of regulating the primaryvoltage includes an act of regulating the primary voltage such that theload voltage approximates and substantially remains below thepredetermined threshold load voltage.
 80. The method of claim 78,wherein the act of regulating includes an act of regulating the primaryvoltage such that the activation signal has a duty cycle of less thanapproximately 1%.
 81. The method of claim 73, wherein the act ofregulating the primary voltage includes an acts of: generating a varyingreference voltage based on the detectable indication; and regulating theprimary voltage based on the varying reference voltage.
 82. The methodof claim 81, wherein the act of generating the varying reference voltageincludes an act of generating the varying reference voltage as anessentially triangular waveform.
 83. The method of claim 81, wherein theact of generating the varying reference voltage includes an act ofgenerating the varying reference voltage as an essentially saw-toothwaveform.
 84. The method of claim 81, wherein the act of generating thevarying reference voltage includes an act of generating the varyingreference voltage such that the varying reference voltage has afrequency in a range of from approximately 5 Hz to approximately 40 Hz.85. The method of claim 81, wherein the energy system fuirther includesat least one capacitor coupled to the primary winding to form a primaryresonant circuit, and wherein the act of regulating includes an act of:controlling a width of pulses input to the primary resonant circuit soas to control the input power to the primary resonant circuit based onat least the varying reference voltage.
 86. The method of claim 85,wherein the act of controlling a width of pulses includes an act ofcontrolling the width of the pulses such that the primary amplitude isapproximately equal to a predetermined amplitude that is proportional tothe varying reference voltage.
 87. The method of claim 86, wherein theact of controlling the input power includes an act of regulating theprimary amplitude based on at least the detectable indication.
 88. Themethod of claim 87, wherein: the act of generating a detectableindication includes an act of generating at least one surge in theprimary amplitude; and the act of detecting the detectable indicationincludes an act of detecting the at least one surge in the primaryamplitude.
 89. The method of claim 88, wherein the act of generating atleast one surge includes acts of: making a comparison of the loadvoltage to the predetermined threshold load voltage; and generating theat least one surge based on the comparison.
 90. The method of claim 89,wherein the act of generating the at least one surge based on thecomparison includes acts of: diverting the output power provided by thesecondary winding away from the variable load when an activation signalis generated; and generating the activation signal when the load voltageexceeds the predetermined threshold load voltage.
 91. The method ofclaim 90, wherein the act of regulating the primary amplitude includesan act of regulating the primary amplitude such that the load voltageapproximates and substantially remains below the predetermined thresholdload voltage.
 92. The method of claim 90, wherein the act of regulatingincludes an act of regulating the primary amplitude such that theactivation signal has a duty cycle of less than approximately 1%.
 93. Anapparatus for use in an energy transfer system, the energy transfersystem including a power supply electrically coupled to a primarywinding, and a secondary winding magnetically coupled to the primarywinding, the primary and secondary windings for transferring power fromthe power supply to a load that is electrically coupled to the secondarywinding, the primary winding and the secondary winding not having afixed spatial relationship to each other, the apparatus comprising: asecondary circuit, electrically coupled to the secondary winding, tomonitor a measurable quantity associated with the load and to provide adetectable indication based on the monitored measurable quantity; and aprimary circuit, electrically coupled to the primary winding, to monitorthe detectable indication provided by the secondary circuit and toregulate a primary voltage across the primary winding based on thedetectable indication so as to regulate a load voltage across the load.94. The apparatus of claim 93, wherein the primary circuit regulates theprimary voltage such that the secondary circuit has a relatively smallactivation duty cycle to provide the detectable indication, so as toreduce a power dissipation in the secondary circuit.
 95. The apparatusof claim 94, wherein the primary circuit regulates the primary voltagesuch that the secondary circuit has an activation time of less thanapproximately 100 microseconds.
 96. The apparatus of claim 95, whereinthe primary circuit regulates the primary voltage such that thesecondary circuit has an activation cycle in a range of fromapproximately 2 milliseconds to 3 milliseconds between consecutiveactivations of the secondary circuit to provide the detectableindication.
 97. The apparatus of claim 94, wherein the primary circuitregulates the primary voltage such that activation duty cycle of thesecondary circuit is less than approximately 1%.
 98. The apparatus ofclaim 94, wherein the primary circuit regulates the primary voltage suchthat the activation duty cycle of the secondary circuit is less thanapproximately 0.5%.
 99. The apparatus of claim 94, wherein the primarycircuit regulates the primary voltage such that the activation dutycycle of the secondary circuit is less than approximately 0.1%.
 100. Theapparatus of claim 93, wherein the secondary circuit is activated toprovide the detectable indication when the measurable quantityassociated with the load exceeds a predetermined threshold level. 101.The apparatus of claim 100, wherein the primary circuit regulates theprimary voltage such that the measurable quantity approximates andsubstantially remains below the predetermined threshold level.
 102. Theapparatus of claim 101, in combination with the primary winding and thesecondary winding.
 103. The apparatus of claim 101, wherein the power istransferred from the power supply to the load via a power channel,wherein a magnetic field that couples the primary and secondary windingsforms a portion of the power channel, and wherein the secondary circuitgenerates the detectable indication on the power channel that indicateswhen the measurable quantity exceeds the predetermined threshold level.104. The apparatus of claim 103, wherein the primary circuit monitorsthe power channel and regulates the primary voltage based on thedetectable indication.
 105. The apparatus of claim 104, wherein theprimary circuit regulates the primary voltage such that the measurablequantity approximates and substantially remains below the predeterninedthreshold level notwithstanding at least one of variations in the loadand variations in a relative position of the primary winding and thesecondary winding.
 106. The apparatus of claim 105, wherein the primarycircuit includes a detector circuit coupled to the primary winding todetect the detectable indication on the power channel.
 107. Theapparatus of claim 106, wherein the primary circuit increases a primaryamplitude of the primary voltage if the detector circuit does not detectthe detectable indication and decreases the primary amplitude if thedetector circuit detects the detectable indication.
 108. The apparatusof claim 106, wherein: the detectable indication includes at least onesurge in the primary voltage; and the detector circuit includes a surgedetector.
 109. The apparatus of claim 108, wherein the secondary circuitincludes: a comparator to make a comparison of the measurable quantityand the predetermined threshold level; and a surge generator coupled tothe comparator to generate the at least one surge based on thecomparison.
 110. The apparatus of claim 109, wherein the surge generatorincludes a shunt circuit that diverts the output power provided by thesecondary winding away from the load when an input signal to the shuntcircuit reaches a shunt activation level, the comparator providing theinput signal having the shunt activation level when the measurablequantity exceeds the predetermined threshold level.
 111. The apparatusof claim 110, wherein the primary circuit regulates the primary voltagesuch that the input signal to the shunt circuit approximates andsubstantially remains below the shunt activation level.
 112. In atranscutaneous energy transfer system that includes a power supplyelectrically coupled to a primary winding and a secondary windingmagnetically coupled to the primary winding, the primary and secondarywindings for transferring power from the power supply to a load that iselectrically coupled to the secondary winding, the primary winding andthe secondary winding not having a fixed spatial relationship to eachother, a method comprising acts of: making a comparison of a measurablequantity associated with the load and a predetermined threshold level;activating a secondary circuit to provide a detectable indication basedon the comparison; and regulating a primary voltage across the primarywinding based on the detectable indication.
 113. The method of claim112, wherein the act of regulating includes an act of regulating theprimary voltage such that the secondary circuit has a relatively smallactivation duty cycle to provide the detectable indication, so as toreduce a power dissipation in the secondary circuit.
 114. The method ofclaim 113, wherein the act of regulating the primary voltage includes anact of regulating the primary voltage such that the secondary circuithas an activation time of less than approximately 100 microseconds. 115.The method of claim 113, wherein the act of regulating the primaryvoltage includes an act of regulating the primary voltage such that thesecondary circuit has an activation duty cycle of less thanapproximately 1%.
 116. The method of claim 413, wherein the act ofregulating the primary voltage includes an act of regulating the primaryvoltage such that the secondary circuit has an activation duty cycle ofless than approximately 0.5%.
 117. The method of claim 113, wherein theact of regulating the primary voltage includes an act of regulating theprimary voltage such that the secondary circuit has an activation dutycycle of less than approximately 0.1%.
 118. The method of claim 112,wherein the act of activating a secondary circuit to provide adetectable indication based on the comparison includes an act ofactivating the secondary circuit to provide the detectable indicationwhen the measurable quantity associated with the load exceeds apredetermined threshold level.
 119. The method of claim 118, wherein theact of regulating aprimary voltage includes an act of regulating theprimary voltage such that the measurable quantity approximates andsubstantially remains below the predetermined threshold level.
 120. Themethod of claim 119, wherein the act of regulating the primary voltageincludes an act of regulating the primary voltage such that themeasurable quantity approximates and substantially remains below thepredetermined threshold level notwithstanding at least one of changes inthe load and changes in a relative position of the primary winding andthe secondary winding.
 121. The method of claim 119, wherein the poweris transferred from the power supply to the load via a power channel,wherein a magnetic field that couples the primary and secondary windingsforms a portion of the power channel, and wherein the method flirtherincludes an act of: generating the detectable indication on the powerchannel that indicates when the measurable quantity exceeds thepredetermined threshold level.
 122. The method of claim 121, wherein theact of regulating the primary voltage includes acts of: monitoring thepower channel; detecting the detectable indication; and regulating theprimary voltage based on the detected indication.
 123. The method ofclaim 122, wherein the act of regulating includes acts of: increasing aprimary amplitude of the primary voltage if the detector circuit doesnot detect the detectable indication; and decreasing the primaryamplitude if the detector circuit detects the detectable indication.124. The method of claim 123, wherein: the act of generating thedetectable indication includes an act of generating at least one surgein the primary amplitude; and the act of detecting the detectableindication includes an act of detecting the at least one surge in theprimary amplitude.
 125. The method of claim 124, wherein the act ofgenerating at least one surge includes acts of: making a comparison ofthe load voltage to the predetermined threshold load voltage; andgenerating the at least one surge based on the comparison.
 126. Themethod of claim 125, wherein the act of generating the at least onesurge based on the comparison includes acts of: diverting the outputpower provided by the secondary winding away from the load when anactivation signal is generated; and generating the activation signalwhen the measurable quantity exceeds the predetermined threshold level.127. An energy transfer system for transferring power from a powersupply located on a first side of a physical boundary to a variable loadlocated on a second side of the physical boundary, the energy transfersystem comprising: a primary winding electrically coupled to the powersupply to generate a magnetic field based on input power provided by thepower supply; a secondary winding magnetically coupled to the primarywinding to receive at least a portion of the magnetic field generated bythe primary winding, the secondary winding providing output power to thevariable load based on the received magnetic field, the magnetic fieldforming a portion of a power channel between the primary winding and thesecondary winding to transfer at least some of the power from the powersupply to the variable load; and a first control circuit, electricallycoupled to the primary winding, to regulate a load voltage across thevariable load based on information related to the variable load that isobtained via the power channel.
 128. The system of claim 127, fiiterincluding a second control circuit, electrically coupled to thesecondary winding, to encode the power channel with at least some of theinformation.
 129. The system of claim 128, wherein the second controlcircuit encodes the power channel with information related to aproximity of the load voltage across the variable load to apredetermined threshold load voltage.
 130. The system of claim 129,wherein the first control circuit regulates a primary amplitude of aprimary voltage across the primary winding based on the informationobtained via the power channel such that the load voltage approximatesthe predetermined threshold load voltage.
 131. The system of claim 127,wherein the power channel has essentially a constant frequency.
 132. Thesystem of claim 131, wherein the frequency of the power channel isapproximately 300 kHz.
 133. In an energy transfer system fortransferring power from a power supply located on a first side of aphysical boundary to a variable load located on a second side of thephysical boundary, the energy transfer system including a primarywinding electrically coupled to the power supply to generate a magneticfield based on input power provided by the power supply, the energytransfer system further including a secondary winding magneticallycoupled to the primary winding to receive at least a portion of themagnetic field generated by the primary winding, the secondary windingproviding output power to the variable load based on the receivedmagnetic field, the magnetic field forming a portion of a power channelbetween the primary winding and the secondary winding to transfer atleast some of the power from the power supply to the variable load, amethod comprising acts of: monitoring the power channel to obtaininformation related to the variable load; and regulating a load voltageacross the variable load based on the information obtained via the powerchannel.
 134. The method of claim 133, further including an act ofencoding the power channel with at least some of the information. 135.The method of claim 134, wherein the act of encoding includes an act ofencoding the power channel with information related to a proximity ofthe load voltage across the variable load to a predetermined thresholdload voltage.
 136. The method of claim 135, wherein the act ofregulating the load voltage includes an act of regulating a primaryamplitude of a primary voltage across the primary winding based on theinformation obtained via the power channel such that the load voltageapproximates the predetermined threshold load voltage.
 137. The methodof claim 133, wherein the power channel has essentially a constantfrequency.
 138. The method of claim 137, wherein the frequency of thepower channel is approximately 300 kHz.